In a number of applications, there is a desire to locate one or more emitters, arranged for emitting electromagnetic energy such as radio transmissions, TV and radar, for example at microwave frequencies via an antenna. Normally this energy is in the form of so-called radio frequency, which refers to that portion of the electromagnetic spectrum in which electromagnetic energy can be generated by alternating current fed to an antenna. Today, a number of methods are employed.
Old emitter location systems use AOA (Angle Of Arrival), while present emitter location systems use TDOA (Time Difference Of Arrival) and FDOA (Frequency Difference Of Arrival).
The devices used for emitter location are in the following referred to as receiving units.
In prior art FIG. 1, a top view of a landscape is shown, where a first emitter 1, and a second emitter 2 are shown. There is furthermore a first receiving unit 3 and a second receiving unit 4. The distance between the first emitter 1 and the first receiver 3 is D13, the distance between the first emitter 1 and the second receiver 4 is D14, the distance between the second emitter 2 and the first receiver 3 is D23 and the distance between the second emitter 2 and the second receiver 4 is D24.
The signal transmitted by the first emitter 1 is u1(t) and the signal transmitted by the second emitter 2 is u2(t). The corresponding received signal, received by the first receiving unit 3, is expressed as u1(t−D13/c0)+u2(t−D23/c0), where t is time and c0 is the speed of light in the medium present for propagation. The corresponding received signal, received by the second receiving unit 4, is expressed as u1(t−D14/c0)+u2(t−D24/c0).
When regarding the first emitter 1 only, the received signal, received by the first receiving unit 3, is u1(t−D13/c0), and the received signal, received by the second receiving unit 4, is u1(t−D14/c0). These received signals are then fed into a correlation calculation algorithm according to the following:c(τ)=∫u1(t−D13/c0)u1(t−D14/c0+τ)dt  (1)where c(τ) is the correlation at the time difference τ. A corresponding correlation calculation may be formulated for the second emitter 2 as well.
In prior art FIG. 2, a graphical representation of c(τ) is shown. Peaks will occur at τ1=(D14−D13)/c0 and at τ2=(D24−D23)/c0. On the τ-axis, the difference between the distances D14−D13 and D24−D23 between an emitter and the respective receiving units is indicated by means of the τ-position of the peak. If the peak occurs at a certain time difference τ, that time may be used for calculating a corresponding difference in physical distance; D14−D13=c0·τ1 and D24−D23=c0·τ2.
In this example, for the first emitter 1, the difference between the distances between the emitter 1 and the respective receiving units 3, 4 is zero; D13=D14, i.e. τ1=0, which is indicated by a corresponding continuous line function 5, where a peak 6 occurs at a τ-value τ1=0. For the second emitter 2, the difference between the distances between the emitter 2 and the respective receiving units 3, 4 is not zero; D23 ≠D24, i.e. τ2≠0, which is indicated by a corresponding dashed line function 7, where a peak 8 occurs at a τ-value τ2≠0.
In prior art FIG. 3, a top view of the first receiving unit 3 and the second receiving unit 4 is shown. For different emitter positions, calculations of equation (1) give rise to different values of τ. As stated previously, a specific time difference τ corresponds to a certain physical distance, constituting the difference in physical distance between the receiving units and the emitter. For a specific time difference τ, a corresponding hyperbola, where the focal points are located at the receiving units, may be calculated. It is a geometrical fact that such a difference in physical distance may be drawn as a hyperbola, where all points of the hyperbola constitute that certain difference in physical distance. In other words, an emitter, giving rise to said certain time difference τ is located along one specific hyperbola.
In FIG. 3, a number of hyperbolas drawn with a continuous line are shown, where a first hyperbola 9 corresponds to the value τ=τb, a second hyperbola 10 corresponds to the value τ=τa, a third hyperbola 11 corresponds to the value τ=0, a fourth hyperbola 12 corresponds to the value τ=−τa and a fifth hyperbola 13 corresponds to the value τ=−τb. The third degenerate hyperbola 11 corresponds to the case where there is no difference between the distances between the emitter in question and the respective receiving unit 3, 4.
There is a problem, however, since it only is indicated that an emitter is located along a certain hyperbola, not where on said hyperbola.
In a further prior art example, attempting to overcome this problem, an emitter 14 and three receiving units, a first receiving unit 15, a second receiving unit 16 and a third receiving unit 17, are positioned as shown in the top view prior art FIG. 4, it is possible to perform the calculation according to equation (1) pair-wise for the receiving units. Each pair-wise calculation results in a certain τ, which in turn gives rise to a certain hyperbola.
The first receiving unit 15 and second receiving unit 16 give rise to a first continuous line hyperbola 18, the first receiving unit 15 and third receiving unit 17 give rise to a second dashed-line hyperbola 19 and the second receiving unit 16 and third receiving unit 17 give rise to a third dot-dashed-line hyperbola 20. The hyperbolas 18, 19, 20 intersect in a first intersection point 21, a second intersection point 22, a third intersection point 23, a fourth intersection point 24, a fifth intersection point 25 and a sixth intersection point 26.
Where those hyperbolas 18, 19, 20 intersect, it is most likely that the emitter in question 14 is positioned. In FIG. 4, the emitter 14 is not exactly positioned along any of the hyperbolas 18, 19, 20, and therefore not exactly at any one of the intersection points 21, 22, 23, 24, 25, 26, which is the most likely scenario in reality, due to measurement inaccuracies.
In short, the correct location of a single emitter is the crossing of all hyperbola lines obtained from each pair of receivers.
Instead of time correlation, it is possible to perform frequency correlation, which provides similar results for moving targets and/or moving receiving units. The major difference is that the simple hyperbolic curve shape will be replaced by a complex quadratic curve shape.
Examples of systems using frequency correlation are disclosed in “Electronic Warfare Target Location Methods” by Richard A Poisel, ISBN 1-58053-968-8, chapter 3.2.2, Differential Doppler”, page 174-175.
There is, however, a problem with this approach, since, as shown in FIG. 4, there are a number of intersection points, and it may be difficult to sort out “false” intersections. In the example according to FIG. 4, there are six intersection points 21, 22, 23, 24, 25, 26, where none exactly corresponds to the exact location of the emitter 14.
Using the approach above can in other words result in several possible crossings, false and true ones, especially if many emitters are present simultaneously.
This results in an association problem between correlation peaks calculated with data from different pairs of receivers, that increases in complexity as the number of simultaneous emitters increases. Another problem is that so-called multipath propagation will result in false emitter detections and locations. Multipath occurs when a transmitted signals is reflected, for example in a lake, resulting in two different paths with different delays for the same original signal.
Special geometries can be even more difficult to analyze, for example if the intersection takes place where the hyperbolas in question are close to parallel.
The prior art examples above have only discussed the two-dimensional case. In the three-dimensional case, for example if an emitter is air-borne, a three-dimensional rotational hyperbolic surface is calculated instead, defined by the two-dimensional hyperbola as it rotates along a line which passes through those two receiving units which are committed at the moment.